Double Sided NMOS/PMOS Structure and Methods of Forming the Same

ABSTRACT

A chip includes a dielectric layer having a top surface and a bottom surface, a first semiconductor layer overlying and bonded to the top surface of the dielectric layer, and a first Metal Oxide-Semiconductor (MOS) transistor of a first conductivity type. The first MOS transistor includes a first gate dielectric overlying and contacting the first semiconductor layer, and a first gate electrode overlying the first gate dielectric. A second semiconductor layer is underlying and bonded to the bottom surface of the dielectric layer. A second MOS transistor of a second conductivity type opposite to the first conductivity type includes a second gate dielectric underlying and contacting the second semiconductor layer, and a second gate electrode underlying the second gate dielectric.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.15/684,533, entitled “Double Sided NMOS/PMOS Structure and Methods ofForming the Same,” filed on Aug. 23, 2017, which is a continuation ofU.S. patent application Ser. No. 14/878,312, entitled “Double SidedNMOS/PMOS Structure and Methods of Forming the Same,” filed on Oct. 8,2015, now U.S. Pat. No. 9,754,844 issued Sep. 5, 2017, which applicationis a divisional of U.S. patent application Ser. No. 14/044,643, entitled“Double Sided NMOS/PMOS Structure and Methods of Forming the Same,”filed on Oct. 2, 2013, now U.S. Pat. No. 9,165,829 issued Oct. 20, 2015,which applications are incorporated herein by reference.

BACKGROUND

In present semiconductor technology, Complementary Metal-OxideSemiconductor (CMOS) devices, such as n-type Metal oxide Semiconductor(MOS) transistors and p-type MOS transistors, are typically fabricatedon semiconductor wafers. The semiconductor wafers are typically siliconwafers. Most of today's semiconductor devices are built on siliconsubstrates having a (100) crystal orientation, which substrates arereferred to as (100) substrates.

Electrons are known to have high mobility in silicon substrate having a(100) silicon crystal orientation, and holes are known to have highmobility in silicon substrate having a (110) crystal orientation.Typically, the electron mobility in a (100) silicon substrate is roughlytwo times to about four times higher than the hole mobility values inthe same substrate. On the other hand, the hole mobility in a (110)silicon substrate is about two times higher than that in a (100) siliconsubstrate. Therefore, the p-type MOS transistors formed on a (110)surface will exhibit significantly higher drive currents than the p-typeMOS transistors formed on a (100) surface. Unfortunately, the electronmobility on (110) surfaces are significantly degraded compared to thaton (100) surfaces.

In addition, p-type MOS transistors and n-type MOS transistors havedifferent preferences regarding the strains. The performance of a MOStransistor can be enhanced through a stressed-surface channel. Thistechnique allows the performance of a MOS transistor to be improved at aconstant gate length, without adding complexity to circuit fabricationor design. Research has revealed that a tensile stress in thechannel-length direction can improve the performance of n-type MOStransistors, and a compressive stress in channel length-direction canimprove the performance of p-type MOS transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 8 are cross-sectional views of intermediate stages inthe manufacturing of a first chip/wafer in accordance with someexemplary embodiments;

FIGS. 9 through 13 are cross-sectional views of intermediate stages inthe bonding of the first chip/wafer to a second chip/wafer and thecontinued formation of the first chip/wafer in accordance with someexemplary embodiments; and

FIG. 14 illustrates a flow chart for forming a package in accordancewith some embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A package with a chip that includes P-type Metal-Oxide Semiconductor(PMOS) transistors and N-type Metal-Oxide Semiconductor (NMOS)transistors and the method of forming the same are provided inaccordance with various exemplary embodiments. The intermediate stagesof forming the package are illustrated. The variations of theembodiments are discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.

FIG. 14 illustrates flow chart 300 for forming a package in accordancewith some embodiments, wherein the process steps illustrated in FIGS. 1through 13 are discussed referring to the steps shown in FIG. 14. FIGS.1 through 8 are cross-sectional views of intermediate stages in themanufacturing of parts of a first chip/wafer in accordance with someexemplary embodiments. Referring to FIG. 1, wafer 100 is provided. Wafer100 includes substrate 101, which is a Semiconductor-On-Insulator (SOI)substrate. Substrate 101 includes semiconductor layer 102, dielectriclayer 104 over semiconductor layer 102, and semiconductor layer 106 overdielectric layer 104. Dielectric layer 104 may be formed of siliconoxide, and hence is referred to as Buried Oxide (BOX) layer 104,although it may also be formed of other dielectric materials.

Semiconductor layers 102 and 106 either have different compositions (forexample, including different elements), and/or have different crystalsurface orientations. In accordance with some embodiments, one ofsemiconductor layers 102 and 106 has the characteristics preferred forforming NMOS transistors, and less preferred for forming PMOStransistors, and the other one of semiconductor layers 102 and 106 hasthe characteristics preferred for forming PMOS transistors, and lesspreferred for forming NMOS transistors.

In some embodiments, semiconductor layers 102 and 106 are crystallinesilicon layers that have different crystalline orientations. In someexemplary embodiments, semiconductor layer 106 is formed of crystallinesilicon having a (100) surface orientation, and semiconductor layer 102is formed of crystalline silicon having a (110) surface orientation.Accordingly, semiconductor layer 106 is preferred for forming NMOStransistors, and less preferred for forming PMOS transistors, whilesemiconductor layer 102 is preferred for forming PMOS transistors, andless preferred for forming NMOS transistors. In alternative embodiments,one or both of the semiconductor layers 102 and 106 is formed of asemiconductor material that is different from crystalline silicon, suchas silicon germanium, silicon carbon, III-V compound semiconductors, orthe like. In some embodiments, semiconductor layer 106 is lightly dopedwith a p-type impurity such as boron, indium, or the like, to a p-typeimpurity concentration between about 1E14/cm³ and about 1E17/cm³.

BOX layer 104, although referred to as an oxide layer, may be anon-oxide dielectric layer. For example, BOX layer 104 may comprise asilicon-containing dielectric material such as silicon nitride, siliconoxynitride, or the like, with its thickness denoted as T1. Semiconductorlayer 102 and BOX layer 104 may be in direct contact with each other,and semiconductor layer 106 and BOX layer 104 may be in direct contactwith each other. In some embodiments, the bonds between semiconductorlayer 102 and BOX layer 104 include silicon-to-oxide bonds, and thebonds between semiconductor layer 106 and BOX layer 104 includesilicon-to-oxide bonds.

Next, referring to FIG. 2, isolation regions 108 are formed insemiconductor layer 106. Isolation regions 108 may be Shallow TrenchIsolation (STI) regions that extend from the top surface ofsemiconductor layer 106 into semiconductor layer 106. STI regions 108may also have bottoms contacting the top surface of BOX layer 104. STIregions 108 may comprise silicon oxide, and may be formed by etchingsemiconductor layer 106 to form recesses, and then filling the recesseswith the respective dielectric material.

FIG. 3 illustrates the formation of NMOS transistors 110 (step 302 inFIG. 14). In some embodiments, NMOS transistor 110 includes gateelectrode 112, gate dielectric 114, and source and drain regions 116,which are heavily doped n-type regions. Source/drain regions 116 mayhave an n-type impurity concentration in the range between about1E19/cm³ and 1E21/cm³, although different impurity concentrations may beused. In some embodiments, source/drain regions 116 extend to the bottomsurface of semiconductor layer 106 to contact the top surface of BOXlayer 104. In alternative embodiments, source/drain regions 116 extendto an intermediate level of semiconductor layer 106. In accordance withsome embodiments, semiconductor layer 106 is used for forming NMOStransistors, and no PMOS transistors are formed on semiconductor layer106. In accordance with alternative embodiments, a small percentage, forexample, less than about 5 percent (or less than about one percent) ofall MOS transistors on semiconductor layer 106 are PMOS transistors, andothers are NMOS transistors. These PMOS transistors may be thetransistors that do not demand high performance.

Dielectric layer 120 is formed over NMOS transistors 110. Aplanarization such as a grinding may be performed to level the topsurface of dielectric layer 120. In some embodiments, dielectric layer120 is a sacrificial layer that is to be replaced in subsequent steps.In alternative embodiments, dielectric layer 120 includes Contact EtchStop Layer (CESL) and Inter-Layer Dielectric (ILD) over the CESL, whichCESL and ILD are left in the final package. Dielectric layer 120 maycomprise silicon carbide, silicon oxide, silicon nitride,Phospho-Silicate Glass (PSG), Boro-Silicate Glass (BSG), Boron-DopedPhospho-Silicate Glass (BPSG), Tetraethyl Orthosilicate (TEOS) oxide, ormulti-layers thereof.

Referring to FIG. 4, hydrogen implantation 124 is performed to implanthydrogen ions into semiconductor layer 102 (step 304 in FIG. 14), sothat hydrogen-concentrated layer 126 is formed in semiconductor layer102. The implantation is performed from the topside (the side facing upin FIG. 4) of wafer 100 in some embodiments. Hydrogen-concentrated layer126 extends into, and is spaced apart from, the bottom surface of BOXlayer 104 by silicon layer 128. Although silicon layer 128 includes someof the hydrogen ions/atoms that are left in silicon layer 128 whenhydrogen ions penetrate through silicon layer 128, the concentration ofhydrogen ions in silicon layer 128 is significantly lower than inhydrogen-concentrated layer 126.

After the hydrogen implantation, wafer 100 is mounted on carrier 130(step 306 in FIG. 14), as shown in FIG. 5. The side of wafer 100 withNMOS transistors 110 faces toward carrier 130. Carrier 130 may be aglass carrier, a ceramic carrier, or the like. In some embodiments,wafer 100 is mounted on carrier 130 through adhesive 132, which may bean Ultra Violet (UV) glue.

Next, wafer 100 is cleaved to separate the portion overlyinghydrogen-concentrated layer 126 and the portion underlyinghydrogen-concentrated layer 126 (step 308 in FIG. 14), so thatsemiconductor layer 102 is thinned. FIG. 6 illustrates the resultingstructure. The respective method is sometimes referred to as a smartcut, which may include annealing wafer 100 at an elevated temperaturesuch as about 350° C. to about 450° C., and/or applying sidewaymechanical forces in the directions parallel to the top surface of wafer100. After the smart cut, a planarization such as a Chemical MechanicalPolish (CMP) is performed to remove the remaining hydrogen-concentratedlayer 126, and to level the top surface of semiconductor layer 128. As aresult, semiconductor layer 128 is left over BOX layer 104. In someexemplary embodiments, semiconductor layer 128 has thickness T2 smallerthan about 3 μm.

Next, referring to FIG. 7, isolation regions (such as STI regions) 144are formed in semiconductor layer 128. STI regions 144 may extend fromthe top surface of semiconductor layer 128 to the top surface of BOXlayer 104 in some embodiments. PMOS transistors 136 are also formed(step 310 in FIG. 14) on semiconductor layer 128. PMOS transistors 136may include gate electrodes 138, gate dielectrics 140, and source anddrain regions 142, which are heavily doped p-type regions. Source/drainregions 142 may have a p-type impurity concentration in the rangebetween about 1E19/cm³ and 1E21/cm³. In some embodiments, source/drainregions 142 extend to the bottom surface of semiconductor layer 128 tocontact the top surface of BOX layer 104. In alternative embodiments,source/drain regions 142 extend to an intermediate level ofsemiconductor layer 128. In accordance with some embodiments,semiconductor layer 128 is for forming PMOS transistors, and no NMOStransistor is formed on semiconductor layer 128. In accordance withalternative embodiments, a small percentage, for example, less thanabout 5 percent (or less than about one percent) of all MOS transistorson semiconductor layer 128 are NMOS transistors. These NMOS transistorsmay be the transistors that do not demand high performance.

Referring to FIG. 8, source/drain silicide regions 146 are formed at thetop surfaces of source/drain regions 142. Source/drain contact plugs 148are formed over and in contact with source/drain silicide regions 146.Gate contact plugs, which are also marked using reference notations 148,are also formed to electrically connect to gate electrodes 138.Furthermore, interconnect structure 150 is formed (step 312 in FIG. 14).Interconnect structure 150 includes dielectric layers 152, and metallines 154 and vias 156 formed in dielectric layers 152. Dielectriclayers 152 may include an Inter-Layer Dielectric (ILD), Inter-MetalDielectric (IMD) layers, passivation layers, and the like. The ILD layerand the IMD layers may be low-k dielectric layers in some embodiments,which have dielectric constants (k values) lower than about 3.5, lowerthan about 3.0, or lower than about 2.5. Dielectric layers 152 may alsoinclude non-low-k dielectric materials having dielectric constants (kvalues) equal to or greater than 3.8. Metal lines 154 and vias 156 mayinclude copper, aluminum, nickel, tungsten, or alloys thereof. Metallines 154 and vias 156 interconnect the integrated circuit devices suchas transistors 136, and may electrically couple the integrated circuitdevices to the overlying metal features 160.

In some embodiments, interconnect structure 150 further includes surfacedielectric layer 158, which is formed at the surface of wafer 100.Surface dielectric layer 158 may be a silicon-containing dielectriclayer, which may comprise silicon oxide (SiO₂), silicon oxynitride(SiON), silicon nitride (SiN), or the like. Metal pads 160 may be formedin surface dielectric layer 158, and may be electrically coupled to MOStransistors 136 through metal lines 154 and vias 156. Metal pads 160 mayalso be formed of copper, aluminum, or alloys thereof. The top surfaceof surface dielectric layer 158 and the top surfaces of metal pads 160may be substantially coplanar with each other.

FIG. 9 illustrates the bonding of wafer 100 to wafer 200 (step 314 inFIG. 14). Wafer 200 may be selected from a device wafer, an interposerwafer, and the like. In the illustrated FIG. 9, wafer 200 includessubstrate 202, P-well region 204, N-well region 208, active devices222/232 (which may include NMOS and PMOS transistors), dielectric layers245, and interconnect structure 244, which include metal lines and viasin dielectric layers. Interconnect structure 244 may also includesurface dielectric layer 250 and metal pads 252 as surface features.

Carrier 130 and glue layer 132 are then removed, and the respectivestructure is shown in FIG. 10. Dielectric layer 120 is thus exposed. Inthe embodiments dielectric layer 120 is a sacrificial layer, dielectriclayer 120 is removed, and is replaced with a CESL and an ILD (alsodenoted as 120). Otherwise, dielectric layer 120 is left un-removed, asalso shown in FIG. 10. Next, as shown in FIG. 11, source/drain silicideregions 164 are formed at the top surfaces of source/drain regions 116,and contact plugs 165 are formed over and in contact with source/drainsilicide regions 164, and to connect to gate electrodes 112. Contactplugs 165 penetrate through dielectric layer 120, which may include aCESL and an ILD in some embodiments.

Referring to FIG. 12, Inter-Layer Vias (ILV) 166 are formed toelectrically connect to metal line/pads 154 (step 316 in FIG. 14).Throughout the description, ILVs 166 are also referred to asthrough-vias. Isolation layer 168 may be formed to encircle each of ILVs166, so that ILVs 166 are electrically decoupled from semiconductorlayers 106 and 128. Although FIG. 12 illustrates that ILVs 166 penetratethrough STI regions 108 and 144, some or all of ILVs 166 may alsopenetrate through semiconductor layers 106 and 128 without going throughSTI regions. In the embodiments all ILVs 166 penetrates through STIregions 108 and 144, but not through semiconductor layers 106 and 128,isolation layers 168 may not be formed (although they can also beformed). In these embodiments, the sidewalls of ILVs 166 are in contactwith STI regions 108 and 144. The process for forming ILVs 166 mayinclude etching dielectric layer 120, semiconductor layers 128 and 106(and/or STI regions 108/144), BOX layer 104, and dielectric layer(s) 152to form openings, filling an isolation layer in the openings, andremoving the bottom portions of the isolation layer in the openings toexpose metal lines/pads 154. A metallic material is then filled into theopenings, followed by a CMP. The remaining portions of the isolationlayer and the metallic material in the openings form isolation layers168 and ILVs 166, respectively.

FIG. 13 illustrates the formation of interconnect structure 174 (step318 in FIG. 14). Interconnect structure 174 may include dielectriclayers 176, and metal lines 178 and vias 180 in dielectric layers 176.Dielectric layers 176 may include low-k dielectric layers, passivationlayers, and the like. The low-k dielectric layers may have dielectricconstants (k values) lower than about 3.5, lower than about 3.0, orlower than about 2.5. Dielectric layers 176 may also include non-low-kdielectric materials having dielectric constants (k values) equal to orgreater than 3.8. Metal lines 178 and vias 180 may include copper,aluminum, nickel, tungsten, or alloys thereof. Through ILVs 166, thedevices on the opposite sides of BOX layer 104 are interconnected toform an integrated circuit.

In some embodiments, Under-Bump Metallurgies (UBMs) 184 are formed, andelectrical connectors 186 are formed on UBMs 184. Electrical connectors186 may include solder balls, copper bumps, or the like. Electricalconnectors 186 may be electrically coupled to the integrated circuitdevices (such as NMOS transistors 110 and PMOS transistors 136) in wafer100 and the integrated circuit devices 222/232 in wafer 200, forexample, through ILVs 166.

After the formation of the bonded wafers 100 and 200, a die-saw may beperformed, and wafer 100 and 200 are sawed into a plurality of packages400, each including one of chips 100′ in wafer 100, and one of chips200′ in wafer 200. Packages 400 may then be bonded to other packagecomponents such as interposers, package substrates, printed circuitboards, or the like.

As shown in FIG. 13, NMOS transistors 110 and PMOS transistors 136 areformed on the semiconductor layers that are bonded to the oppositesurfaces of the same BOX layer 104. In these embodiments, BOX layer 104may be formed of a homogenous material, with no layered structure suchas metal layers and vias embedded therein. Furthermore, except ILVs 166and isolation layers 168, there may not exist other conductive materialsin BOX layer 104. ILVs 166 and isolation layers 168 may havesubstantially straight edges that are substantially perpendicular to thetop surface and the bottom surfaces of BOX layer 104. ILVs 166 andisolation layers 168 continuously penetrate through semiconductor layers106 and 128 and BOX layer 104, with no discontinuity therein.

In the above-discussed embodiments, as shown in FIG. 1, semiconductorlayer 106, which has the (100) surface orientation, is over BOX layer104, and is used for forming NMOS transistors. Semiconductor layer 102and the respective semiconductor layer 128 having the (110) surfaceorientation, on the other hand, are used for forming PMOS transistors.In alternative embodiments, semiconductor layer 106 may have the (110)surface orientation, and is used for forming PMOS transistors with noNMOS transistors thereon. Semiconductor layer 102 and the respectivesemiconductor layer 128 cut from it, on the other hand, may have the(100) surface orientation, and are used for forming NMOS transistorswith no PMOS transistors thereon.

The embodiments of the present disclosure have several advantageousfeatures. PMOS transistors and NMOS transistors are formed on differentsemiconductor layers, and are inter-coupled through ILVs to formintegrated circuits. Accordingly, the semiconductor layers may beselected and/or tuned to improve the performance of the respective PMOStransistors and NMOS transistors formed thereon. For example, thecrystalline orientations and the strains in the semiconductor layers maybe tuned to improve the performance of the respective MOS transistors.

In accordance with some embodiments, a chip includes a dielectric layerhaving a top surface and a bottom surface, a first semiconductor layeroverlying and bonded to the top surface of the dielectric layer, and afirst MOS transistor of a first conductivity type. The first MOStransistor includes a first gate dielectric overlying and contacting thefirst semiconductor layer, and a first gate electrode overlying thefirst gate dielectric. A second semiconductor layer is underlying andbonded to the bottom surface of the dielectric layer. A second MOStransistor of a second conductivity type opposite to the firstconductivity type includes a second gate dielectric underlying andcontacting the second semiconductor layer, and a second gate electrodeunderlying the second gate dielectric.

In accordance with other embodiments, an integrated circuit structureincludes an oxide layer having a top surface and a bottom surface, afirst silicon layer overlying and bonded to the oxide layer, and asecond silicon layer underlying and bonded to the oxide layer. The firstsilicon layer has a (100) surface orientation, and the second siliconlayer has a (110) surface orientation. An NMOS transistor includes afirst gate dielectric overlying the first silicon layer, a first gateelectrode overlying the first gate dielectric, and a first source/drainregion in the first silicon layer. A second silicon layer is underlyingand bonded to the oxide layer, wherein the second silicon layer has a(110) surface orientation. A PMOS transistor includes a second gatedielectric underlying the second silicon layer, a second gate electrodeunderlying the second gate dielectric, and a second source/drain regionin the second silicon layer. A through-via penetrates through the oxidelayer, the first silicon layer, and the second silicon layer.

In accordance with yet other embodiments, a method includes forming afirst transistor at a top surface of an SOI substrate, wherein the SOIsubstrate includes a first semiconductor layer, a dielectric layer overand bonded to the first semiconductor layer, and a second semiconductorlayer over and bonded to the dielectric layer. The first transistor isformed on the second semiconductor layer. The method further includesthinning the first semiconductor layer, and forming a second transistoron the first semiconductor layer. The first transistor and the secondtransistor are of opposite conductivity types. A through-via penetratesthrough the first semiconductor layer, the dielectric layer, and thesecond semiconductor layer.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: forming a first transistorat a surface of a first semiconductor layer of a substrate; performing ahydrogen implantation, wherein hydrogen ions penetrate through the firsttransistor and form a hydrogen-concentrated layer in a bulksemiconductor region, with the bulk semiconductor region being under thefirst semiconductor layer; performing a cleaving process to separate thebulk semiconductor region into a second semiconductor layer and a thirdsemiconductor layer, wherein the second semiconductor layer remainsbeing in the substrate after the cleaving process; and forming a secondtransistor on the second semiconductor layer.
 2. The method of claim 1further comprising performing a planarization process on the secondsemiconductor layer after the cleaving process.
 3. The method of claim 1further comprising: attaching the substrate to a carrier; and forming asource/drain silicide region at a surface of a source/drain region ofthe second transistor, wherein the first transistor is between thesource/drain silicide region and the carrier.
 4. The method of claim 1,wherein the first transistor and the second transistor are of oppositeconductivity types.
 5. The method of claim 1 further comprising forminga through-via to connect the first transistor to the second transistor,wherein the through-via comprises a first portion level with the firsttransistor, and a second portion level with the second transistor. 6.The method of claim 5 further comprising forming an isolation layerencircling the through-via.
 7. The method of claim 5, wherein the firstportion of the through-via is wider than the second portion of thethrough-via.
 8. A method comprising: forming first isolation regionspenetrating through a first semiconductor layer to contact a buriedisolation layer, wherein the buried isolation layer is further overlyinga semiconductor region; forming a first transistor based on a firstactive region in the first semiconductor layer; forming a dielectriclayer covering the first transistor; performing a hydrogen implantation,wherein a hydrogen-concentrated layer is formed to separate thesemiconductor region into a second semiconductor layer and a thirdsemiconductor layer, wherein the second semiconductor layer contacts theburied isolation layer; removing the hydrogen-concentrated layer and thethird semiconductor layer from the second semiconductor layer; formingsecond isolation regions penetrating through the second semiconductorlayer to contact the buried isolation layer; and forming a secondtransistor based on a second active region in the second semiconductorlayer.
 9. The method of claim 8, wherein the removing thehydrogen-concentrated layer and the third semiconductor layer comprises,after the hydrogen implantation, performing a cleaving process tomechanically separate the second semiconductor layer from the thirdsemiconductor layer.
 10. The method of claim 9, wherein the cleavingprocess is performed when the dielectric layer is on the firsttransistor.
 11. The method of claim 8 further comprising, after thehydrogen implantation, performing a planarization process on the secondsemiconductor layer, wherein the second isolation regions are formedafter the second semiconductor layer is planarized.
 12. The method ofclaim 8, wherein a first source region and a first drain region of thefirst transistor contacts the buried isolation layer.
 13. The method ofclaim 12, wherein a second source region and a second drain region ofthe second transistor contacts the buried isolation layer.
 14. Themethod of claim 8, wherein a first gate of the first transistor overlapsa second gate of the second transistor.
 15. The method of claim 8,wherein the first semiconductor layer and the second semiconductor layerhave different surface orientations.
 16. A method comprising: formingfirst isolation regions penetrating through a first semiconductor layerto contact a buried isolation layer; forming a first transistor on thefirst semiconductor layer; forming a dielectric layer covering the firsttransistor; after the first transistor is covered with the dielectriclayer, thinning a semiconductor region underlying the buried isolationlayer to form a second semiconductor layer, wherein the secondsemiconductor layer contacts the buried isolation layer; forming secondisolation regions penetrating through the second semiconductor layer tocontact the buried isolation layer; forming a second transistor on thesecond semiconductor layer; and interconnecting the first transistor andthe second transistor.
 17. The method of claim 16, wherein theinterconnecting comprises forming a through-via to connect the firsttransistor to the second transistor, wherein the through-via comprises afirst portion level with the first transistor, and a second portionlevel with the second transistor.
 18. The method of claim 17, whereinthe forming the through-via comprises etching the dielectric layer, theburied isolation layer, and at least one of the first isolation regionsand the second isolation regions.
 19. The method of claim 18, whereinthe through-via comprises a first portion penetrating through the firstsemiconductor layer, and a second portion penetrating through one of thefirst isolation regions.
 20. The method of claim 16, further comprising:forming a first source/drain silicide region on a surface of a firstsource/drain region of the first transistor, wherein the firstsource/drain silicide region is formed after the second transistor isformed; and forming a second source/drain silicide region on a surfaceof a second source/drain region of the second transistor.